Electroless metal deposition or plating is useful for the deposition of a metal or mixture of metals on a non-conductive or dielectric surface in the absence of an electric source. Plating on non-conductive or dielectric substrates is used in a variety of industries, including decorative plating and electronic device fabrication. One of the main applications is the manufacture of printed circuit boards. The electroless deposition of a metal on a substrate usually requires pretreatment or sensitization of the substrate surface to make the surface catalytic to the deposition process. Various methods have been developed to catalyze the substrate.
U.S. Pat. No. 3,011,920 discloses a method to catalyze a substrate by immersion of the substrate in the colloidal catalyst solution prepared by palladium ions with stannous ions to form a palladium-tin colloid. This method requires a step of acceleration after catalyzing the substrate surface, whereby the catalyst core is exposed. U.S. Pat. No. 3,904,792 discloses an improvement of colloidal palladium-tin catalysts to provide catalysts in a less acidic environment. Hydrochloric acid is replaced partially by another soluble salt of the acid. Such a palladium-tin catalyst system still presents a number of limitations. The outer shell of the catalyst colloid SnCl42− is easily oxidized, thus the catalyst particle grows in size and loses its catalytic surface area dramatically. In addition, because of environmental requirements, a tin-free catalyst for electroless plating was developed.
With good bonding to palladium, nitrogen-containing polymers were widely described in the literature to synthesize palladium nanoparticles as catalysts. U.S. Pat. No. 4,725,314 discloses a process for the preparation of a catalytic adsorbate in aqueous solution using an organic suspending agent such as polyvinyl pyrrolidone to protect a colloid. US 20120097548A discloses the use of glutathione and palladium forming a stable aqueous solution of nanoparticles, which may catalyze electroless metal deposition. US 20120145555A discloses the use of N-containing heterocyclic compounds such as histidine or urocanic acid to catalyze electroless metal deposition. However, nitrogen containing polymers not only had good interaction with palladium but also with copper. This bonding was difficult to remove and the interconnection defect problem resulted in the unreliability of electronic devices. Also, such catalysts usually worked in alkaline medium. Copper surfaces of innerlayer or base copper are easily oxidized in such alkaline medium. This causes interconnection defect problems.
U.S. Pat. No. 4,652,311A discloses the use of polyacrylic acid and polyacrylamide as suspending agents. Palladium ions are reduced by mild reducing agents in the presence of suspending agents. To stabilize against further growth of particles, the addition of a soluble alcohol solution is needed. The alcohol is more likely to drive the completion of reduction and stabilize the particles against aggregation. It works at an alkaline range and the palladium concentration ranges from 10 to 2,000 ppm. WO2011/030638A discloses the use of polyacrylic acid as a dispersing agent, which is able to avoid aggregation and sedimentation of palladium colloid. Catechol is used to suppress the palladium oxidation while a copper-acid inhibiting agent is used to limit the generation of copper colloid or copper hydroxide.
However, a colloidal catalyst system which has a balanced bath stability, adsorption capability and catalytic activity at the same time is still desired.